Recently, there has been an increasing interest in energy storage technology. As the application fields of energy storage technologies have been extended to mobile devices such as cellular phones, camcorders and notebook computers, as well as electric motors, the demand for batteries as a power source to have high energy density has been increasing. Lithium secondary batteries are considered as the best one capable of satisfying such a demand, and the researches thereof have been actively made.
Among secondary batteries currently used, a lithium secondary battery developed in the early 1990's comprises an anode made of carbon materials capable of intercalating or disintercalating lithium ions, a cathode made of lithium-containing oxides, and a non-aqueous electrolyte solution obtained by dissolving a suitable amount of lithium salt in a mixed organic solvent.
As the anode active material of the lithium secondary battery, various carbon-based materials, including artificial graphite, natural graphite, and hard carbon which can intercalate and disintercalate lithium ions have been used. Among these carbon-based materials, graphite has low discharge voltage of −0.2V relative to lithium, so a battery using graphite as an anode active material exhibits high discharge voltage of 3.6V. Therefore, such a graphite active material has been the most widely used since it can provide advantages in term of the energy density of a lithium battery and also has good reversibility to ensure the long life time of the lithium secondary battery. However, the graphite active material has low density (theoretical density 2.2 g/cc) in the preparation of an electrode to provide low capacity, which is unfavorable in terms of energy density per unit volume of the electrode, and also it is apt to react with organic electrolyte adversely at high discharge voltage, which may result in ignition or explosion by the abnormal operation, overcharging of the battery and so on.
Recently, as the use of lithium secondary batteries expands, there are gradually increasing demands for a lithium secondary battery capable of maintaining good performances under severe conditions such as high temperature and/or low temperature and being stably charged even at high voltage.
Meanwhile, it is possible to improve the capacity characteristic of a lithium secondary battery by changing carbon-based anode active materials with non-carbon-based materials such as silicon oxide. However, some anode materials including silicon oxide are irreversible. Therefore, some anode materials intercalate lithium ions at the first charge and cannot disintercalate about 20% of the lithium ions in the later discharge. Accordingly, about 20% of cathode active materials used in the first charge cannot be involved in the following charge and discharge after the first charge, and eventually the efficiency of the lithium secondary battery is lowered.
In order to solve this problem, there has been attempted to prepare a nanoparticle composite consisting of a carbon-based material and a silicon-based material and use it as an anode active material. Such a nanoparticle composite can improve the capacity retention ratio of a battery to a degree by means of the carbon-based material acting as an electrical conductor. However, the carbon-based material should be excessively present in an amount more than 50 wt % in the nanoparticle composite in order to provide relatively good capacity retention to the battery, which reduces a capacity of the battery. Further, although the carbon-based material present in an excessive amount as disclosed in the above, there is still a problem that a capacity on or after the 50th cycle is lowered less than 1500 mAh/g.
Therefore, there is a need to develop a lithium secondary battery that can improve the decrease of a capacity retention ratio during initial cycles when a non-carbon-based material is used as an anode material.